The present invention relates to a magnetic recording medium comprising a nonmagnetic support and a magnetic layer comprising a ferromagnetic hexagonal ferrite powder dispersed in a binder on at least one surface of the nonmagnetic support, the magnetic recording medium having a low noise and extremely excellent electromagnetic conversion characteristics.
In the field of magnetic discs, 2MB MF-2HD floppy discs using Co-modified iron oxide have been generally loaded in personal computers. However, in these days where the data volume to be treated is abruptly increasing, it cannot be said that the capacity of these floppy discs is sufficient. Thus, it has been demanded to realize high capacity floppy discs.
Further, in the field of magnetic tapes, in recent years, as personal computers or workstations diffuse, magnetic tapes for storing computer data as an external storage medium (so-called backup tapes) are eagerly studied. In putting magnetic tapes having such utility into practical use, especially as not only computers become small-sized, but also information throughput increases, improvements in storage capacity have been strongly demanded in order to achieve an increase of the storage capacity and a reduction in size.
Hitherto, it has not been suggested that durability is enhanced by using an aramid base in a tape-like medium of a magnetic recording medium containing iron oxide, Co-modified iron oxide, CrO2, a ferromagnetic metal powder, or a hexagonal ferrite powder.
Media prepared by applying a magnetic layer dispersed in a binder on a nonmagnetic support are widely used. Of these media, those using a ferromagnetic metal fine powder or a hexagonal ferrite fine powder are known excellent in high density recording characteristics. In the case of discs, examples of high capacity discs using a ferromagnetic metal fine powder having excellent high density recording characteristics include 10 MB MF-2TD and 21 MB MF-2SD, and examples of high capacity discs using hexagonal ferrite include 4 MB MF-2 ED and 21 MB Floptical. However, it could not be said that even these discs are sufficient with respect to the capacity and performance.
Under these circumstances, there have been made many attempts to enhance the high density recording characteristics. For example, LS-120 and ZIP discs realize high density recording with a high capacity of from 100 MB to 120 MB. In addition, high density recording with an areal recording density of 0.2 Gbit/inch2 or more is being demanded. The areal recording density is expressed by the product of linear recording density and track density. With respect to currently commercially available discs with a storage capacity of 100 MB class, it is necessary to increase both the linear recording density and the track density several times. For keeping stable recording/reproduction, such high capacity magnetic recording media are required to have a higher dimensional stability than that of the conventional media.
Now, magnetic heads with electromagnetic induction being an operating principle (induction type magnetic heads) have hitherto been used and diffused.
However, in using them in a higher density recording/reproduction region, limits initiate to be seen. That is, for obtaining a high reproducing output, it is necessary to increase the number of coil turns of a reproducing head. However, in this case, there was involved a problem that the inductance increases, and the resistance increases at a high frequency, resulting in a reduction in the reproducing output.
In recent years, reproducing heads with magnetic resistance (MR) being an operating principle are proposed and initiate to be used in hard discs, etc. Further, application of such heads to magnetic tapes is proposed in JP-A-8-227517. The MR heads provide a reproducing output of several times higher than the induction type magnetic heads. Further, since the MR heads do not use an induction coil, they lower an instrumental noise such as impedance and reduce a noise of the magnetic recording media, thereby enabling to obtain a high S/N ratio. In another word, if a noise of the magnetic recording media that has hitherto hided in the instrumental noise is decreased, it becomes possible to undergo good recording/reproduction and abruptly enhance the high density recording characteristics.
For reducing the noise, there may be considered various means. Especially, it is effective to decrease the size of ferromagnetic powder particles. In recent magnetic powders, are used ferromagnetic hexagonal ferrite fine powders having a tabular size of 40 nm or less.
Further, there are surface properties adapted to the MR heads, and the conventional surface designs may not make the MR heads sufficiently exhibit the characteristics. JP-A-10-302243 describes a coating type magnetic recording medium having a low noise and excellent high density characteristics in a recording/reproduction system combined with an MR head. However, even such a coating type magnetic recording medium was not always satisfactory.
Accordingly, the invention is to provide a magnetic recording medium having good electromagnetic conversion characteristics, especially a markedly improved CN ratio in a high density recording region, and excellent productivity, and especially a coating type magnetic recording medium combined with an MR head, which is low in the noise in a recording/reproduction process.
The invention has been attained by the following means.
(1) A magnetic recording medium comprising a support having thereon a magnetic layer composed mainly of a ferromagnetic powder and a binder, wherein 100/900 xcexcm2 or less protrusions having a height, as measured by AFM, of 30 nm or more are present on the surface of the magnetic layer, the magnetic layer has a magnetic switching volume of from 0.1xc3x9710xe2x88x9217 to 5xc3x9710xe2x88x9217 mL and an Hc of 2,000 Oe (159.2 KA/m) or more, and the surface of the support in the magnetic layer forming plane side has a PSD in the machine direction, as measured by an interferometric roughness tester with a magnification of 5 times, of from 500 to 4,000 xcexcm3 at a space wavelength of 10 xcexcm.
(2) A magnetic recording/reproduction process comprising reproducing the magnetic recording medium as set forth above in (1) by an MR head.